Bulk Handling Australia has developed its own brand of Static Protected FIBC woven PP fabrics EX-STATIC. Part of BHA’s continuous development program was to develop such a fabric to cater for the growing needs of industry to protect their workers from hazardous work environments.
Static electricity is one of the insidious sources of fire and explosion encountered in modern industry. It is by nature unpredictable and therefore difficult to detect. In some industry sectors it is viewed almost as a black art.
Electrostatic Hazards associated with FIBC's
Whenever powders, granular material or pellets are handled, static electricity is invariably produced. Each single particle of a product being handled may only carry a small amount of electrostatic energy. However, when the particles are bulked together, such as in an FIBC, the cumulative effect of all the individually charged particles can be great.
In industrial situations involving FIBC's there are principally three ways in which potentially hazardous levels of electrical charge can be generated. When filling an FIBC the product transferred to the bag is often in an electrostatically charged condition. Bulking the product within the bag intensifies the volume charge density resulting in a high electric field radiating from the bag walls.
Discharging the product from the bag can also generate charge due to tribo-electrification (frictional charging) between the product and the bag wall. Once electrostatic charge has been generated or accumulated by any of the mechanisms described above, a situation can arise in which electrostatic discharges (ESD's) can occur. Finally, simply handling, cleaning or rubbing the outside surface of the bag may, under certain conditions, generate relatively high levels of static electricity.
Brush discharge is a discrete electrostatic discharge from an insulator such as a plastic surface. Brush discharges are low in energy having a maximum energy content of around 4 millijoules.
Propagating Brush discharge (PBD)- a highly energetic discharge produced by the breakdown of an electrical double layer (capacitor). Propagating Brush discharge can occur when a thin electrically insulating sheet, such as the fabric of an FIBC, is polarised with charges of one sign appearing on one surface and charges of opposite sign appearing on the other surface.
In practice, propagating brush discharges may transfer at least 1000 millijoules of energy) and is accompanied by a bright flash. The power of such discharges is high and presents an ignition hazard to flammable solvents and dust clouds.
If PBD's can be ruled out and the maximum energy content of a brush discharge is 4 milliJoules (mJ), is reasonable to conclude that the FIBC design is safe for flammable atmospheres with a minimum ignition energy in excess of 4 mJ. A general recommendation arising from this is that such bags are safe for use with flammable dusts, but should not be used in the presence of flammable hydrocarbon vapours.
Types of FIBC’s
From the point of view of electrostatic ignition hazard, FIBC's can be categorised into four distinct types by their mode of construction.
Type A -Type A bags have no special electrostatic safety features and are, therefore, not recommended for use with sensitive flammable dusts and powders. In addition, they should not be filled or discharged in environments where fine dust clouds and/or flammable solvent vapours are present.
Type B -Type B FIBC's are similar to Type A and may be made from plain (non-antistatic) polypropylene weave. In the case of Type B, however, the wall fabric exhibits a breakdown voltage of 4 kilovolts or less
Type C-Type C FIBC's are specifically designed for sensitive flammable environments including flammable hydrocarbon solvent atmospheres. They are constructed from electrically conductive fabric or standard fabric which has an electrically conductive or antistatic coating. Alternatively, a standard fabric is used which contains conductive threads
Type D -The Type D classification for FIBC's essentially refers to those bags which are claimed to have antistatic or static dissipative properties without the requirement of earthing. There are a number of Type D designs currently commercially available, most (but not all) of which incorporate fine partially-conductive threads in the weave of the fabric. Unlike the Type C designs the threads, although generally parallel, are not interconnected. This type of FIBC is likely to have an applied static-dissipative coating.
BHA Type D EX-STATIC TM fabrics incorporate a unique static dissipative yarn interwoven with the PP tapes. The yarn is made up of twisted long staple fibers and has electrical properties permanently died into the fibers. It has an ultra low capacitance eliminating the possibility of an incendiary discharge from the yarn itself.
Combined with a high performance antistatic additive in the PP polymer creates good static protection. These built-in mechanisms are weather proof and stand up to normal wear and tear even after many uses of the FIBC. The performance of this fabric is well inside the 0.4 milli Joules criteria for high hazard environments.